Don't listen to him; I'm alreayd thinking "Hmm.. elemental elven demiplanes, eh? Hmm... could definitely use that one. Drow transformation? Nice... ooh! Pan-Yggdrasiloid unification! Should have thought of that myself."
But in all, I definitely know what you mean about finding an element that ties everything together. It's a great feeling and it makes for some awesome games.
I agree with rycanada, I know exactly what you're talking about.
Write everything you have so far down, as you just did, and then leave your ideas to simmer for a while. It's the only option I know of. If someone else has any tips, i'd like to hear them.